Excitability changes induced in the striatal dopamine-containing terminals following frontal cortex stimulation

The excitability of the dopamine-containing terminal field in the striatum (St) following frontal cortex (FC) stimulation was investigated in halothane-anesthetized rats. Either glutamic acid (GLU, 6.2 mM) or square pulses (a train of 25 pulses, 500-800 microA/0.3 ms: 1 Hz/20 s) were used to stimulate FC. To stimulate St monophasic square wave pulses (10-4000 microA/0.5 ms/1 Hz) were delivered. Excitability was measured by determining the threshold for antidromic activation of substantia nigra cells. Threshold was defined as the minimum current required for antidromic invasion of the cell on 100% of non-collision trials. The mean threshold current was 1029 +/- 167 microA. Following FC stimulation a significant decrease (30%) in excitability was observed in most cases (80%). No correlation between firing rate and threshold fluctuations was observed. It is concluded that FC activity decreases the excitability of the dopaminergic nigrostriatal terminal field. Whether this is a direct or an indirect effect is discussed.